Communications systems which employ wireless transceivers are well known. However, as is the case with most electronic technologies today, there is an ever-increasing demand to improve information transmission rates and reduce power consumption while, at the same time, reducing the influence of noise and improving the quality of transmission. A fixed transmission power limits the transmission rate. The longer a transmission takes, the more power is consumed. Therefore, it is desirable to decrease the amount of time a given transmission takes. Additionally, different transmission rates affect the quality of the transmission. For example, higher transmission rates are more susceptible to noise. Increased noise results in decreased transmission rates.
Current communications systems control the transmission rate and the transmission power independently using two (2) control variables. One power control algorithm monitors network conditions. In response, the power control algorithm updates the power control variable at a regular interval. A separate algorithm monitors channel conditions to adjust the data rate. Neither algorithm can guarantee the highest possible transmission rate at the lowest possible transmission power, nor do the algorithms minimize the level of interference generated to other devices.
Wireless devices tend to interfere with one another due to reasons such as lack of mutual protocol, use of an unlicensed frequency band or use of a collision based access mechanism. An example of interference generated by lack of a mutual protocol is the simultaneous co-located use of Bluetooth and IEEE 802.11b devices. Neither device is able to directly communicate with the other, and their differing protocols result in packet collisions, which in turn results in lost data on both the Bluetooth and IEEE 802.11b network. The use of unlicensed bands such as the 2.4 GHz ISM band or the 5 GHz U-NII band also results in devices that must contend for the wireless medium in an interference environment. In addition, some protocols, such as the IEEE 802.11 Distributed Coordination Function (DCF) channel access mechanism, yield environments where even IEEE 802.11 devices must tolerate interference from one another.
A wireless communications device must have a sufficiently large signal to noise ratio to decode a received packet correctly. Noise from various sources affects the receiver. This noise can be thermal noise that is generated in the receiver itself or it can be noise external to the receiver. Noise external to the receiver may come from other devices in the area. The amount of noise received from such devices depends on the amount of power such a device transmits, the path loss to the receiver and the amount of power transmitted by the interfering device. The amount of interference is thus reduced if the interfering device transmits less power and transmits less frequently.
Any wireless communications device is a potential interferer to other devices. To reduce the interference generated, the wireless device can reduce its transmit power level. However, there is a level below which the transmit power cannot be reduced, because below this level the device that is being transmitted to will no longer be able to decode the transmission successfully. Thus it is desirable to transmit at or just above this threshold to minimize the amount of interference that other devices in the area experience as a result of the transmission.
In addition to power control, wireless devices may also control the rate at which they transmit. The rate of transmission controls the length of time that the wireless device must transmit. In packet-based systems such as wireless local area networks, transmission time includes of some fixed overhead typically including a preamble and header, and the payload. Thus the transmission time, tTX, is equal totTX=t0+Nb/Rwhere t0 is the overhead time, Nb is the number of data bits in the packet and R is the transmission rate. Reduction of the amount of time, i.e. tTX, that a wireless device transmits reduces the amount of interference to other devices. Increasing the transmission rate, R, reduces the transmission time. As the transmission rate, R, is increased, the received SNR threshold for the receiving device to properly decode the packet also increases. Depending on the transmit power and path loss, the transmitting device may or may not be able to transmit at its maximum rate, as the receiving device may not be able to decode at that rate. Wireless devices can control one or both of their transmission rate, R, and their transmit power level.
As illustrated by FIG. 1, in typical wireless communications devices, an adaptive method 110 controls the transmission rate (R) 120 based on inputs 130 such as the measured packet error rate (PER) 130c, signal to interference noise ratio (SINR) 130b and signal to noise ratio (SNR) 130a. In such wireless communication devices, e.g. 802.11 wireless LANs, the transmit power is fixed. At best, the transmit power may be changed via a user interface. It is not adaptively changed.
It is desirable in view of the foregoing to provide a solution that enables higher transmission rates at lower power levels and minimizes the level of interference generated. The present invention controls both the transmission rate and power level in a fashion that minimizes the level of interference generated by the act of transmission. The present invention enables enhanced coexistence of wireless devices via a joint transmit power and rate control scheme. By employing the present invention, wireless devices may reduce the level of mutual interference, thereby allowing improved performance.